Apple's iPad may be a blockbuster hit, but its success won't stop Android-based slates from leading the market within three years, according to a new IDC study. While Apple dominates the tablet market today, the iPad's share of the worldwide market is declining as lower-priced competitors like the Amazon Kindle Fire arrive. Apple shipped 15.4 million iPads in the fourth quarter of 2011, up from 11.1 million in the third quarter. The company had 54.7 percent of the worldwide tablet market in Q4, down from 61.5 percent in Q3, the IDC report says. Android's share is rising, largely at the expense of the iPad, which runs Apple's iOS mobile operating system. "As the sole vendor shipping iOS products, Apple will remain dominant in terms of worldwide vendor unit shipments," said Mainelli. "However, the sheer number of vendors shipping low-priced, Android-based tablets means that Google's OS will overtake Apple's in terms of worldwide market share by 2015." There's one very bright spot for Apple, however. IDC predicts that iOS will remain the market leader, revenue-wise, through 2016 and beyond. Kindle Fire's Success The biggest tablet development last year was the emergence of Amazon as a major player. The online retailer shipped 4.7 million units of its Kindle Fire tablet, enough to take second place with 16.8 percent of the worldwide market. "Amazon's widely-reported entry into the media tablet market with a $199, 7-inch product seemed to raise consumers' awareness of the category worldwide despite the fact that the Fire shipped almost exclusively in the U.S. in the fourth quarter," said IDC analyst Tom Mainelli in a statement. The Fire's success was the driving force behind Android's market-share rise: up from 32.3 percent in the third quarter of 2011, to 44.6 percent in the fourth. (The Fire runs a custom version of Android.) Samsung's share grew slightly from 5.5 percent in Q3 to 5.8 percent in Q4. And while Barnes & Noble increased its tablet shipments, its market share actually slipped in Q4 to 3.5 percent, down from 4.5 percent. In the fourth quarter of 2011, worldwide tablet shipments rose 56.1 percent to 28.2 million units--a dramatic 155-percent increase from Q4 2010. Strong consumer demand has led IDC to bump up its 2012 tablet-shipment forecast to 106.1 million units, up from its earlier forecast of 87.7 million.